Conventionally, there has been provided, for example, a leakage signal cancellation circuit for use in an interrogator of a RFID system, or the like, which divides a part of a transmitted signal by using a coupler, which adjusts the amplitude and phase of the divided signal by using a variable attenuator and a phase shifter in such a manner that the signal has the same amplitude as a leakage signal which leaks from a transmitter to a receiver and is in opposite phase with the leakage signal, and which combines a generated cancel signal with a received signal so as to cancel out the leakage signal (for example, refer to patent reference 1).
There has been also proposed a method of adjusting the amplitude and phase of a signal divided by using a quadrature modulator so as to generate a cancel signal which has the same amplitude as a leakage signal and is in opposite phase with the leakage signal, and combining the cancel signal with a received signal so as to cancel out the leakage signal (for example, refer to nonpatent reference 1).
This reference further shows a circuit which constructs a feedback loop in such a manner as to follow variations in the leakage signal, and which can cancel out the leakage signal with stability (refer to FIG. 2).
This circuit performs the following operation. That is, the circuit converts the received signal into baseband I and Q signals using a quadrature mixer after generating the cancel signal so as to cancel out the leakage signal. A signal which is a part divided from a transmitted signal is inputted to the LO input of this quadrature mixer. The gains of baseband I and Q signals which are outputted by the quadrature mixer are adjusted and are led to the I and Q inputs of the above-mentioned quadrature modulator. Then, the leakage signal which was not able to be canceled out through the combination with the cancel signal is decomposed into I and Q signals by the quadrature mixer using the signal which is the divided part of the transmitted signal. The I and Q signals of the leakage signal which was not able to be canceled out are added to the I and Q signals of the quadrature modulator which generates the cancel signal from the same signal which is the divided part of the transmitted signal, so that the leakage signal is further canceled out. Even when the amplitude and phase of the leakage signal vary and therefore the leakage signal cannot be sufficiently canceled out, the conventional circuit operates in the same way to follow the variations in the leakage signal, so that the leakage signal is further canceled out.
[Patent reference 1] JP,10-62518,A
[Nonpatent reference 1] “Solving the problems of a single antenna frequency modulated CW radar”, written by Beasley et al., Record of the IEEE 1990 International Radar Conference, 7-10 May 1990, pp 391 to 395, FIGS. 1 and 2
The amplitude and phase of the leakage signal which leaks from the transmitter to the receiver vary according to the surrounding environments of the antenna, i.e., whether an object, such as a reflector (metal) or a human body, exists in the vicinity of the antenna. A problem with the conventional leakage signal cancellation circuit shown in patent reference 1 is that because its mechanism for adjusting the signal divided from the transmitter in such a manner that the signal has the same amplitude as the leakage signal and is in opposite phase with the leakage signal is semifixed, the conventional leakage signal cancellation circuit cannot follow variations in the amplitude and phase of the leakage signal which are caused by change in the surrounding environments of the antenna and therefore cannot perform stable cancellation of the leakage signal.
The conventional leakage signal cancellation circuit shown in nonpatent reference 1 can follow variations in the leakage signal with feedback control. However, such a circuit needs to make the leakage signal certainly pass through a quadrature mixer in order to acquire a cancel signal. A problem with the structure shown in nonpatent reference 1 is therefore that because the feedback control works and a large leakage signal is inputted to the quadrature mixer during operation until the leakage signal is sufficiently canceled out, an expensive quadrature mixer which can withstand an input with large signal power must be used as the mixer.
The present invention is made in order to solve the above-mentioned problems, and it is therefore an object of the present invention to provide a leakage signal cancellation apparatus which can acquire a cancel signal certainly and which can implement cancellation of a leakage signal with stability while following variations in the leakage signal.